1. Field of the Invention
This invention relates to a photodiode for an LD/PD module for bidirectional optical communication system with signal light of two wavelengths λ1 and λ2 via a single optical fiber which can suppress the influence of transmitting light upon the photodiode. The single-fiber bidirectional optical communication system using of a single fiber both for transmitting and receiving signals employs an LD/PD module. The LD/PD module has a substrate, a package, an LD emitting transmission signals and a PD receiving signals mounted upon the common substrate in the common package. This is called a multiwavelength bidirectional LD/PD module.
Another optical communication system transmits more than one signal by the light of plural wavelengths in one direction via a single fiber and receives the multiwavelength signals by a PD module having a plurality of photodiodes. The PD module has more than one photodiode on a common substrate in a common package. This is a unidirectional multiwavelength PD module.
A photodiode is a sensor which converts light power (light signals) to a photocurrent (electric signals) in proportion to the light power. The PD is sometimes called an O/E transducer or O/E sensor. The photodiode is a highly sensitive sensor. The LD generates strong light power for transmitting optical signals to a far distanced port. Although the wavelengths are different for transmitting signals and for receiving signals, the photodiode which has sensitivity also to the transmission wavelength has a possibility of sensing the transmission power yielded by the laser diode mounted on the same package.
This application claims the priority of Japanese Patent Applications No. 11-201519 (201519/1999) filed on Jul. 15, 1999 and No. 11-260016 (260016/1999) filed on Sep. 14, 1999 which are incorporated herein by reference.
The phenomenon that the PD senses the transmission signal emitted from the LD which is stored in the same package is called “optical crosstalk”. The LD light of the same port is noise for the PD. Sensing the transmission light at the same port hinders the PD receiving the transmission light from the counterpart port (e.g., central station). It is important to suppress the crosstalk from the LD (transmitting device) to the PD (receiving device) at the same package. There are two interactions between the transmitting device and the receiving device on the same substrate. One is the optical crosstalk which is optical coupling between the PD and the LD. The other is the electric crosstalk which is conveyed from the LD to the PD by electromagnetic waves. Both kinds of crosstalk are difficult for the LD/PD module to conquer. This invention aims at solving the optical crosstalk.
There are several versions of the bidirectional LD/PD module for making use of a single fiber both for transmission and reception with regard to the modes of signal separation of the transmitting light and the receiving light. A typical signal separation device is a WDM (wavelength division multiplexer) which divides the common path spatially to a transmission path and a reception path by the difference of the light wavelengths. The WDM separation alleviates the difficulty of the optical crosstalk, since the WDM allots different paths to the LD and the PD for separating them spatially. A special disposition is a serial alignment of the PD and the LD on the same straight line. In this case, most of the transmission path is common with the reception path. The common path type module is suffering from more serious crosstalk problem.
This application claims the priority of Japanese Patent Applications No.11-201519(201519/1999) filed on 15 Jul., 1999 and No. 11-260016(260016/1999) filed on 14 Sep., 1999, which are incorporated herein by reference.
2. Description of Related Art
FIG. 1 shows a typical multiwavelength bidirectional optical communication system having LD/PD modules at a central station and at a subscriber port. At the station, an LD1 generates downward signals. The downward signals travel through an optical fiber 1, a WDM 2, an optical fiber 3, another WDM 4 and an optical fiber 5 to a PD2 at the subscriber site. The PD2 converts the downward optical signals to electric (received) signals. At the ONU (optical network unit) terminal, a subscriber LD2 generates upward optical signals. The upward signals spread through an optical fiber 6, the WDM 4, the fiber 3, the WDM 2 and the fiber 7 and attain at a PD1 at the station. The PD2 converts the light signal from the subscriber into electric signal. The single fiber 3 enables both the upward signals and the downward signals to spread in both directions between the central station and the ONU terminal. The WDM 2 at the station alternatively allocates the downward signals and the upward signals into the fiber 1 or fiber 7 by the difference of the wavelengths. The downward light wavelength is denoted by λ2. The upward light wavelength is designated by λ1. Both signals are propagating in both directions in the same fiber 3. The WDM 4 at the subscriber port (ONU) alternatively allocates the downward signals and the upward signals into the fiber 5 or fiber 6 by the difference of the wavelengths. The ONU receives the downward (receiving) signals λ2 by the PD2. The LD2 generates the transmission (upward) signals λ1 at the ONU. Electric circuits following the PD2 and the LD2 are omitted in FIG. 1.
The words of “transmitting” or “receiving” signals have reverse directions (or inverse flows) at the station and at the ONU. In the description, the words should be defined at the ONU site. Thus, the upward light λ1 corresponds to the transmitting signals. The downward light λ2 carries the receiving signals. The prior art (multiwavelength bidirectional communication) of FIG. 1 separates the PD2 and the LD2 spatially by dividing the light paths by the WDM.
FIG. 2 shows a prior multiwavelength unidirectional optical communication system for transmitting various signals from a central station in a downward direction to a subscriber port. At the station, an LD1 and an LD2 generate different downward signals of λ1 and λ2. The downward signals travel through an optical fiber 1 or 7, a WDM 8, an optical fiber 3, another WDM 4 and an optical fiber 5 or 6 to a PD1 or a PD2 at the subscriber site. The WDM 4 separates two different signals by the difference of the wavelengths λ1 and λ2. The PD1 senses λ1. The PD2 detects λ2. Crosstalk occurs between PD1 and PD2 also in the multiwavelength unidirectional system.
FIG. 3 is a sectional view of a prior art PD module which has widely been used as a receiving device in the optical communication network having the spatially separated paths as shown in FIG. 1 or FIG. 2. The PD module has a metallic bottom circular stem 10 with lead pins 9 extending downward. A PD chip 12 is mounted via a submount 11 onto the stem 10. A thin metal cap 14 having a lens 13 is adjusted and welded on the stem 10. A cylindrical sleeve 15 is adjusted and welded on the stem 10 above the cap 14. A ferrule 16 is inserted, adjusted and fixed in an axial hole of the sleeve 15. The ferrule 16 clamps an end of an optical fiber 17. The end of the ferrule 16 is slantingly polished. An elastic bend-limiter 18 caps the top end of the sleeve 15 for protecting the fiber 17 from overbending. FIG. 1 and FIG. 2 include LD modules in addition to the PD modules explained by FIG. 3. The LD module is omitted to describe, since it is simply obtained by replacing the PD chip with an LD chip in the module of FIG. 3.
This invention is applicable to the spatially separating LD/PD or PD/PD modules as shown in FIG. 1 or FIG. 2. The prior PD module of FIG. 3 has a three-dimensional structure making use of a metallic package. The expensive metal package hermetically seals the PD device and shields the PD from external noise. Although the metal-can PD module has advantages of the sealing property and the shielding property, such a PD module has drawbacks of the number of parts, the necessity of adjustment, the number of steps of fabrication and the high cost. High cost prevents the metal-can type PD, LD or LD/PD modules from pervading widely.
Flat planar type LD, PD or LD/PD modules have been intensively investigated as low-cost devices. The flat planar type is called “PLC (planar lightguide circuit)”-type. FIG. 4 denotes an example of a PLC type PD module containing a bottom-incidence type PD. This invention can also be applied to the PLC module, which is now explained preliminarily. The PLC module is constructed upon a rectangular silicon bench (Si-bench) 19. The Si-bench 19 has a V-groove 20 formed by selectively etching the Si substrate from an end to a midway point in an axial direction in the middle. A slanting mirror plane 21 is formed by the same etching at the final end of the V-groove 20. A PD chip 23 is mounted just above the slanting mirror plane 21 upon the Si-bench 19. The PD chip 23 is a bottom surface incidence type PD with a light sensing part 24 at the top. An optical fiber 22 is fitted into and fixed to the V-groove 20. The light beam emanating from the fiber 22 travels in the V-groove 20 and shoots the mirror 21. Being reflected upward by the mirror 21, the signal light goes into the PD 23 via the bottom surface and attains to the sensing part 24. A photocurrent flows in proportion to the power of the signal light. The PLC type module succeeds in excluding the adjustment operation. Exclusion of adjustment alleviates the cost by facilitating production.
Both the PD modules of FIG. 3 and FIG. 4 can be applied for detecting the receiving signals separated by the WDMs in FIG. 1 and FIG. 2. The WDM is made, for example, by making a light waveguide branch having wavelength selectivity in a lightguide layer on a Si substrate. WDMs have various types with different shapes, different materials or different principles. FIG. 5 shows a prism type WDM. The WDM has two transparent glass columns 25 and 26 with a rectangular triangle section. The glass columns have a slanting surface coated with a dielectric multilayer 27. The dielectric multilayer 27 has wavelength sensitivity which allows one wavelength to pass through but reflects the other wavelength. The sensitivity enables the WDM to integrate the receiving light and the transmitting light. The reciprocal property of reflection or refraction laws allows the same WDM to act both as a wavelength integrating device and a wavelength separating device. In FIG. 5, the λ2 receiving light going out of a fiber 28 is reflected by the dielectric layer 27 and reaches a PD 30. The transmitting light λ1 made by an LD 29 goes through the dielectric layer 27 and enters the fiber 28. This invention can contribute to such an LD/PD module including a PD and an LD spatially separated by the WDM.
The WDM separating type LD/PD modules, however, are less significant than unseparating path type LD/PD modules for the present invention. The unseparating path type LD/PD modules mean the module which has a PD without an inherent path separated from the LD path. The unseparating type modules have been first proposed by the present Inventors. The unseparated path type LD/PD module aligns a PD and an LD on an axial line which is an extension of the optical fiber. The PD has no exclusive path different from the path of the LD. Alignment of the PD and the LD on the same axial line simplifies the structure of the module. This is a strong point. The same path for the LD and the PD, however, enhances the difficulty of the crosstalk more seriously than the WDM separating type LD/PD module. FIG. 6 denotes an example of the unseparated path type LD/PD which has bigger significance to the present invention. A direct purpose is to solve the problem of the crosstalk in the unseparated path type LD/PD modules of FIG. 6.
In the figure, a dotted rectangle denotes a housing (package) 31. The housing 31 has a Si-bench which is omitted here for simplicity. An optical fiber 32 is mounted in an axial direction in the housing 31. An LD 33 is installed at an extension of the fiber 32 in the housing 31. A WDM filter 35 slantingly cuts the fiber 32 at a spot distanced from the LD 33. The WDM filter 35 discriminates the light by the wavelength. A PD 34 is mounted above the WDM filter. The inherent path from the WDM 35 to the PD 34 is quite short. The LD 33 yields strong transmission light λ1 of about 1 mW or more. The LD-emitted strong light travels to the left in the fiber, passes the WDM 35 and makes a long journey in the fiber 32 to e.g., the station. The receiving light λ2 propagating to the right in the fiber, is reflected by the WDM 35 to the PD 34. A receiving region 36 of the PD 34 detects the receiving light λ2. The transmitting light λ1is strong light. The receiving λ2 is weak light. λ1 and λ2 have a common path from the beginning end to the WDM. The propagating directions are reciprocal for λ1 and λ2 and the WDM 35 separates λ1 and λ2. Sometimes the strong LD light partially invades into the PD 34 due to the short distance between the common axial line and the PD. The invasion of the LD light causes the optical crosstalk. Even a small rate of the LD light raises a large noise in the PD, since the LD power is strong enough and the receiving light is weak.
If a conventional PD as shown in FIG. 7 were used as the PD in FIG. 6, the invasion of the strong LD power would cause more than noise due to the wide range of sensitivity. The LD-noise would prevail over the receiving signal in the PD. The S/N rate (signal/noise rate) would be far smaller than 1. The conventional InP type PD chip is fabricated from an epitaxial wafer having an n-InP substrate 37 and epitaxial layers consisting of an n-InP buffer layer 38, an n-InGaAs light receiving layer 39 and an n-InP window layer 40 epitaxially deposited in turn on the n-InP substrate 37. A p-region 41 is formed by diffusing zinc (Zn) from the center of the n-InP window layer 40. An annular p-electrode 42 is formed on the p-region 41. An antireflection film 43 is made on the p-region 41 within the annular p-electrode 42. A passivation film 44 is deposited on the InP window layer 40 out of the p-electrode 42 for protecting the pn-junction. An n-electrode 45 is deposited on the bottom of the InP substrate 37. In the case of the upper surface incidence type as shown in FIG. 7, the bottom n-electrode 45 is an overall coating metal film and the top p-electrode 42 is annular. In the case of the bottom incidence type, the bottom n-electrode should be annular and the top p-electrode should be an overall metal film.
FIG. 8 shows the sensitivity distribution of the prior InGaAsP-type photodiode (PD) of FIG. 7. A front falling part (P) at shorter wavelengths corresponds to the band gap of the InP window layer 40. The light of a shorter wavelength than P is insensitive to the PD, because the shorter wavelength light is absorbed by the InP window layer 40. Another falling part (R) at longer wavelengths corresponds to the band gap of the InGaAs light receiving layer 39. The light having longer wavelength of lower energy (hν) is insensitive to the PD, because the longer wavelength light is not absorbed by the InGaAs light receiving layer 39. Thus, the conventional InP-type PD has a wide range (Q) of sensitivity from the band gap (P) of the InP window layer 40 to the band gap (R) of the InGaAs light receiving layer 39. The sensitivity range (Q) includes both 1.3 μm and 1.55 μm bands. The prior PD has sufficient sensitivity both for the 1.3 μm and 1.55 μm bands.
The prior PD of FIG. 7 is generally adopted for sensing the infrared light in the optical communication. The PD has a wide sensitivity range from 1 μm to 1.65 μm as shown in FIG. 8. The wide sensitivity range is a strong point of the PD because of the probable common use of the PD both for receiving the 1.3 μm and 1.55 μm light. However, when the wide sensitive PD is used in the LD/PD module, the PD is apt to cause large crosstalk owing to the wide sensitivity range including the 1.3 μm band.
If the transmitting light λ1 had lower energy than the receiving light λ2(λ1>λ2), a contrivance of the light receiving layer would enable the PD to decrease the crosstalk. The PD can sense the light having higher energy than the band gap energy of the light receiving layer by absorbing the light at the light receiving layer and converting the light energy into photocurrent. But the PD is insensitive for the light having energy lower than the band gap of the light receiving layer, since the light passes through the light receiving layer without loss. If the light receiving layer were made of a material having a band gap of an intermediate energy between the transmission light energy and the receiving light energy, the PD would sense only the receiving light exclusively.
If the transmitting light λ1 had higher energy than the receiving light λ2(λ1<λ2), any contrivance of the light receiving layer would fail to decrease the crosstalk. The crosstalk cannot be suppressed by changing the band gap of the light receiving layer at all. Such case is just an object of the present invention. Namely, the transmission wavelength λ1 is shorter than the receiving wavelength λ2 in the present invention (λ1<λ2). For example, the transmission light is λ1=1.3 μm and the receiving light is λ2=1.55 μm. Poor performance of the WDM will induce large crosstalk in the WDM-separated type of FIG. 1 or FIG. 2. The unseparated path type LD/PD module incurs far greater crosstalk due to the short distance between the PD and the axial common signal line.
The reason why the LD/PD module of FIG. 6 is liable to invite large optical crosstalk will be described here. In FIG. 6, all the strong transmitting light emitted from the LD does not go into the optical fiber 32. A part of the laser (LD) light shoots the platform or the resin outside of the fiber due to the wide aperture of the LD light. The extra light is reflected, refracted and scattered by the platform (Si-bench) or the resin. Unfortunately, the silicon (Si-) bench is transparent to the LD transmitting light λ1. The resin enclosing the LD is also transparent to the LD infrared light. This is an origin of the difficulty of the optical crosstalk. The Si bench and the resin reflect, refract and scatter the stray LD light. Scattering direction or scattering power depends upon the distribution of the resin, the shape of the platform or the disposition of the LD or PD. The loci of the scattering, refracting or reflecting light is complex. Whole of the platform seems to glow brightly to the PD. The random LD light out of the fiber is called “stray” light or “scattered” light.
The LD light is invisible to human eyes but sensible to the PD. The stray light λ1 goes into the PD in various directions at various heights as inner noise. The noise LD light invades into the PD via the upper surface, the bottom surface and the side surfaces. Then, the stray LD transmission light causes the crosstalk without entering the optical fiber. Since the stray light does not pass the WDM filter, the stray light cannot be suppressed by an improvement of the performance of the WDM at all. Furthermore, the LD light once going into the fiber sometimes induces the crosstalk due to the leak from the fiber owing to the scattering. An increase of the LD power makes it difficult to suppress the leak of the LD λ1 light. The WDM is indifferent to the stray LD light.
Nobody had been aware of the fact. The skilled has believed that the imperfection of the WDM filter would invite the LD light to invading to the PD. They thought that the requisite should be a contrivance of eliminating the LD light in the short path from the WDM to the PD sensing region. The description of the present invention discriminates two modes of the noise LD light by calling the LD light not entering the fiber “stray light” and the LD light once guided into the fiber but refracted by the WDM to the PD “leak light”.
The Inventors had once contrived a superb PD having a selective absorption layer which does not absorb the λ2 receiving light but absorbs the. LD λ1 light in the epitaxial layers.
{circle around (1)} Japanese Patent Laying Open No. 11-83619 (83619/1999) (Applicant: Sumitomo Electric Industries, LTD., Inventors: Yoshiki Kuhara, Hiromi Nakanishi, Hitoshi Terauchi, filed on Sep. 3, 1997) proposed such a PD. FIG. 9 shows the structure of the PD chip proposed by {circle around (1)}. An n-type InGaAs light receiving layer 47 is grown on an n-type InP substrate 46. A p-type region 48 is made at a central part of the n-InGaAs light receiving layer 47 by diffusion zinc (Zn) which is a p-dopant. A pn-junction denoted by a dotted curved line is formed as an interface between the n-type region and the p-type region. An i-layer (depletion layer) 49 accompanies the pn-junction just below. The p-region 48 has a p-electrode 50 on the top surface. A passivation film 51 (for example, SiN) is made on the n-InGaAs light receiving layer 47 out of the p-electrode 50 for protecting the edge of the pn-junction. An n-type InGaAsP absorption layer 52 is formed on the bottom of the n-InP substrate 46. The InGaAsP absorption layer 52 was the novel point of {circle around (1)}. An annular n-electrode 53 having an opening is formed upon the bottom of the absorption layer 52. The central opening of the n-electrode 53 is an entrance aperture of the PD chip. The entrance aperture is protected by an antireflection window 54 coating the absorption layer 52.
FIG. 10 denotes more a detailed structure of the PD chip suggested by {circle around (1)}. In practice, an n-InP buffer layer 56 is inserted between the n-InP substrate 46 and the n-InGaAs light receiving layer 47 for improving the crystallographic property of the light receiving layer 47. An n-InP window layer 55 is formed upon the InGaAs light receiving layer 47. Zn is thermally diffused from the InP window layer 55 to the InGaAs receiving layer 47 for making the p-region 48. If the PD lacked the InGaAsP absorption layer 52, the PD would have a wide sensitivity range (Q) as explained by referring to FIG. 8 due to the InP substrate 46 and the InP buffer layer 56 which give a tolerant lower limit (P).
The example is a bottom-incidence type PD, for example, applicable to the module of FIG. 6. The novel point is the InGaAsP absorption layer 52 in {circle around (1)}. Quaternary compound InGaAsP allows free choice of the band gap and the lattice constant. Semiconductors or insulators generally can absorb the light of energy higher than the band gap but cannot absorb the light of energy lower than the band gap. Wavelengths are inversely proportional to energy. A band gap wavelength λg is defined as the wavelength of the light having the energy equal to the band gap. Lower energy light (λ>λg) passes the material but higher energy light (λ<λg) is partially or entirely absorbed by the material. Then, if the band gap of the quaternary InGaAsP is chosen as an intermediate value between the transmitting wavelength λ1 and the receiving wavelength λ2, the InGaAsP would not absorb the λ2 but would selectively absorb the λ1 light. This is the basic idea of {circle around (1)}.
For example, if the transmitting light wavelength is λ1=1.3 μm and the receiving light wavelength is λ2=1.55 μm, the band gap of the absorption layer should be about λg=1.40 μm˜1.46 μm. The absorption layer has, e.g., carrier (electron) concentration of 1×1018 cm−3 and a thickness of about 5 μm. This is a considerably thick layer. The carrier concentration is also high. The large thickness (e.g., 5 μm) is required for absorbing all the 1.3 μm light without the rest. The InGaAsP of λg=1.40 μm˜1.46 μm has attenuation coefficient α=104 cm−1 for 1.3 μm light. A 5 μm-thick layer gives attenuation of exp(−αd)=0.007 which means very small transparency of 0.7%. The high electron concentration (1018 cm−3) aims partially at prohibiting the forward resistance from rising due to the absorption layer and partially at facilitating the recombination of the electron-hole pairs yielded by the light. The absorption layer is effective to suppress the optical crosstalk between the PD and the LD, since the absorption layer does not absorb the receiving light λ2 (1.55 μm) but selectively absorbs the LD transmission light (1.55 μm).
FIG. 11 shows the wavelength dependence of the transparency of the InGaAsP absorption layer 52 explained in {circle around (1)}. The compound ratio is selected for giving a band gap wavelength e.g., λg=1.42 μm(λg =1.40 μm˜1.46 μm). The absorption layer absorbs the light of a shorter wavelength than 1.42 μm but allows the light of a longer wavelength than 1.42 μm to pass. The sensitivity is 1A/W for 1.55 μm but less than 0.01A/W for 1.3 μm. The extinction ratio is less than 1/100 (20 dB).
Since the PD is the bottom incidence type, a mixture of the λ1 and λ2 light goes into the PD from the bottom aperture. The λ1 light is fully absorbed by the absorption layer 52. No λ1 light reaches the light receiving region (the depletion layer). The PD senses no λ1 light. The WDM-scattered LD light causes no crosstalk in the improved PD. The PD is, in particular, effective to the unseparated path type LD/PD module as shown in FIG. 6. The PD is also effective to the separated path type LD/PD module of FIG. 1 or FIG. 2, since the absorption layer can annihilate the extra λ1 light which has not been eliminated by the WDM filter.
{circle around (1)} is a superb, excellent invention which provides the n-InP substrate 46 with the n-InGaAs light receiving layer 47 on the upper surface and the n-InGaAsP absorption layer 52 on the bottom surface. Epitaxy makes both the InGaAs light receiving layer 47 and the InGaAsP absorption layer 52. Both surface film formation makes the epitaxy difficult. The allocation of the absorption layer and the light receiving layer on both surfaces brings about an advantage annihilating electron-hole pairs borne by the absorption of λ1 in the absorption layer 52 without making an influence upon the light receiving layer 47. The design of the thickness of the absorption layer required to consider the attenuation factor exp(−αd) for the 1.3 μm light. It was supposed that the absorption layer would annihilate the obstacle λ1 light which escapes from the WDM filter to a enough small value of exp(−αd). The absorption layer was thought to remove the crosstalk between the transmission signals and the receiving signals completely.
The assumption turned out not to be entirely true. {circle around (1)} is effective indeed to annihilate the λ1(1.3 μm) leak which goes via the bottom into the PD, because the light passes the absorption layer. The WDM passing leak light is not the whole of the light emanating from the LD and going into the PD. Besides the WDM passing leak, some part of the LD light attains to the PD after being scattered or reflected at the fiber end, the resin or the substrate several times. This λ1 light is called stray light for discerning it from the WDM-passing leak light. The LD chip has a short resonator length which is equal to the length of the PD chip. The short resonator admits the LD emitting light to spread in a wide aperture both vertically and horizontally because of using no lens. Considerable part of the LD light does not go into the fiber. Fiber-excluded, scattered LD light illuminates the whole of the Si bench brilliantly, in particular, in the unseparating type LD/PD module aligning the PD and the LD on an extension of the fiber axial line as shown in FIG. 6. This fact is beyond the imagination of the skilled. Since the Si bench is transparent to the λ1 light, the λ1 light propagates in the Si bench and the resin. The Si bench and the resin glitter with λ1. Some part of the scattered λ1 attains to the PD without passing the WDM. Enhancement of the WDM performance has no effect on preventing the fiber-excluded λ1 from shooting the PD. It is important to remove the fiber-excluded λ1 scattered (stray) light by the bench, the resin or the package as well as to eliminate the leak λ1 light due to the imperfection of the WDM.
As defined before, the λ1 light emanated from the LD but excluded from the fiber is called stray light. The stray λ1 fills the housing as a whole. Although the PD is the bottom-incidence type, the λ1 light stray does not necessarily come into the PD via the bottom. The PD has a big thickness of about 200 μm. Some λ1 stray light obliquely enters the PD via the sides. Some scattered stray light shoots the top of the PD. Other stray light goes into the PD via the bottom. Various paths guide the stray λ1 light to the PD. The stray λ1 light invading from the sides or the top makes its own way to the depletion layer 49 in the InGaAs receiving layer without touching the absorption layer 52. The stray light generates photocurrent in the depletion layer 49 or the p-region 48. This is noise current for the PD. The noise-generation is called crosstalk. Namely, the bottom absorption layer is impotent to prevent the stray light from entering the PD via the sides or the top. Furthermore it turns out that the side-top entering stray light is rather more powerful than the bottom entering stray light.
For example, it would be possible to widen the p-electrode 50 (in FIG. 9) on the light receiving layer and cover the other part of the receiving layer with an opaque film for suppressing the top entering stray light. It is far difficult to suppress the side-entering stray light. PD chips are made by repeating epitaxy, diffusion, lithography or other wafer processes on a circular wafer and by scribing the wafer into plenty of chips along the cleavage lined lengthwise and crosswise. It is impossible to treat with the sides of the chips any more. The height of the sides is about 200 μm. The wide sides are not protected with any opaque films. The sides are exposed to a jam of the stray light.
The wavelength selectivity of WDM filters or WDM couplers is 15 dB to 20 dB at present. The receiving port requires at least 30 dB or desirably 40 dB of the wavelength selectivity as a whole. For example, if the LD power is 1 mW (=0 dBm), the least receivable power is −30 dBm and the S/N ratio of the noise (λ1) to the signal to the PD is −10 dB, the required whole wavelength selectivity is 40 dB (30 dB+10 dB). Further, if the least receivable (detectable) power is −35 dB and the S/N ratio to the PD is −15 dB (about a thirtieth), the required wavelength selectivity is 50 dB (35 dB+15 dB).